Biomass pretreatment for efficient conversion to sugars remains one of the most challenging obstacles to the design of an economically viable bio-refinery for liquid fuel production. Several pretreatment and biomass fermentation fractionation (separation of carbohydrate and lignin components) technologies exist or under consideration, these include dilute acid, solvo-thermal, hydrothermal, ammonia fiber explosion, and others.
Enzymatic hydrolysis is the rate limiting step in the fermentation of biomass to sugars. The main barrier is the highly crystalline structure of cellulose that limits accessibility to enzyme adsorption sites and slows the hydrolysis of cellulose to sugars in aqueous media. To increase enzyme accessibility, a combination of high temperature and extremes of pH are used during common pretreatment steps like dilute acid or ammonia fiber explosion.
A promising new candidate technology for biomass fractionation is ionic liquid pretreatment. In an ionic liquid pretreatment process, biomass is dissolved in an ionic liquid, and carbohydrates such as hemicellulose and cellulose are precipitated on the addition of an anti-solvent such as water. The regenerated cellulose possesses an amorphous structure that is especially amenable to enzymatic saccharification and may contain little or no inhibitory components for subsequent processing to ethanol or other biofuels. The lignin remains in solution and can be removed from the ionic liquid by other thermal, liquid extraction, or other methods that have not been fully described. The relatively expensive ionic liquid is recovered and recycled into the process.
Ionic liquid pretreatment method has been used to convert lignocellulose to sugars, since current pretreatment approaches are energy and cost intensive. Methods are being developed for the conversion of crystalline cellulose to sugars with improvements in yield and rate of sugar production using a simultaneous pretreatment and saccharification using a one step ionic liquid. The pretreatment step has been developed with the use of ionic liquid to break down crystalline cellulosic biomass. While the second step of hydrolyzing cellulose to sugars would require separation of cellulose from ionic liquid, an enzyme that is compatible with the concentrations of ionic liquid used in pretreatment step would eliminate the need for this step, thereby improving yield and reducing time and cost.
Endoglucanases are hydrolytic enzymes that catalyze the endo cleavage of cellulose polymer to smaller units like cellobiose and are used as a starting point in the hydrolysis of cellulosic polymers to simpler sugars like glucose. While the enzymes, i.e. biocatalysts, discovered in nature have a distinct and characteristic activity, the activity of these natural variants is generally low in industrial application that require high activity to compete with chemical catalysis. Secondly, green chemistry requires the use of products that are biodegradable and enzymes are biodegradable catalysts that can be used in industrial hydrolysis of cellulose to sugars.
Currently, people use techniques that are based on treating the biomass with a combination of high temperature and acid or base, or chemicals like lime. These methods have two distinct disadvantages—first, the industrial enzymes are used to break down cellulose are not compatible with such harsh methods making a single pretreatment and saccharification method impossible. Secondly, these methods create unwanted byproducts that interfere with the downstream hydrolysis and fuel production steps.